The disclosure of Japanese Patent Application No. 2001-327384 filed on Oct. 25, 2001, including the specification, drawings, and abstract is incorporated herein by reference in its entirety.
1. Field of Invention
The invention relates to a vehicular braking control apparatus and, more particularly, to a braking control apparatus for performing frictional braking by means of a frictional braking device and regenerative braking by means of a regenerative braking device. The invention also relates to a braking control method of the braking control apparatus.
2. Description of Related Art
Japanese Patent Application Laid-Open No. 2000-50409 discloses a braking force control apparatus that has a frictional braking device and a regenerative braking device that usually performs braking in a cooperative braking mode by frictional braking and regenerative braking. The braking force control apparatus discloses shifting from the cooperative braking mode to a single braking mode by frictional braking in case of necessity, and that returns from the single braking mode to the cooperative braking mode is known.
According to such a braking control apparatus, a part of the kinetic energy of the vehicle normally lost during braking is usually recovered as electric energy by regenerative braking. The effect of regenerative braking can be reliably eliminated if regenerative braking creates an obstacle to performing other controls of the vehicle, such as an antiskid control.
In general, during a braking control in the cooperative braking mode, a target braking amount for the entire vehicle, such as a braking requirement made by a driver, is distributed into a target frictional braking amount and a target regenerative braking amount. The frictional braking device and the regenerative braking device are controlled such that each of the frictional braking amount and the regenerative braking amount becomes equal to a corresponding one of target braking amounts.
During a shift from the cooperative braking mode to the single braking mode, when the sum of the target frictional braking amount and the target regenerative braking amount is equal to the target braking amount for the entire vehicle is maintained, the target regenerative braking amount and the target frictional braking amount are gradually reduced and increased respectively. The target frictional braking amount is finally set equal to the target braking amount for the entire vehicle. Similarly, if the single braking mode realized by frictional braking, is returned to the cooperative braking mode, when the sum of the target frictional braking amount and the target regenerative braking amount is equal to the target braking amount for the entire vehicle is maintained, the target regenerative braking amount and the target frictional braking amount are gradually increased and reduced respectively until the target regenerative braking amount reaches a maximum possible value.
However, the frictional braking device performs braking through a frictional force that is generated by pressing a frictional member such as a brake pad against a rotating body such as a brake rotor rotating together with wheels. Hence, if a friction characteristic value of the rotating body or the frictional member, such as a friction coefficient of the rotating body or the frictional member, is different from its designed value, the actual frictional braking amount does not become precisely equal to a target braking control amount no matter how precisely the pressing force applied to the rotating body by the frictional member is controlled.
An error in the actual frictional braking amount for the entire vehicle resulting from a deviation in the friction characteristics of the rotating body or the frictional member is small in a situation in which braking is performed in the cooperative braking mode. However, such an error is remarkable during a shift between the cooperative braking mode and the single braking mode realized by frictional braking. Hence, if there is a great deviation in the friction characteristics, the braking amount for the entire vehicle abruptly changes during a shift between the cooperative braking mode and the single braking mode. As a result, the vehicle deceleration abruptly fluctuates, which may cause a sense of incongruity to a passenger in the vehicle.
For example, if it is assumed that the target braking amount for the entire vehicle in the cooperative braking mode is 10, that the ratio between the target frictional braking amount and the target regenerative braking amount is 1:9, and that the actual deviation in the frictional braking amount resulting from a deviation in the friction characteristics of the frictional member is 10%, the actual error in the frictional braking amount for the entire vehicle in the cooperative braking mode is 0.1xe2x80x20.1=0.01, that is, 1%. On the other hand, the actual error in the frictional braking amount for the entire vehicle in the single braking mode is 1xe2x80x20.1=0.1, that is, no less than 10%.
As described above, in the braking control apparatus of the related art wherein braking is usually performed in the cooperative braking mode realized by frictional braking and regenerative braking and wherein a shift between the cooperative braking mode and the single braking mode realized by frictional braking is made in case of necessity, the braking amount for the entire vehicle abruptly changes during the shift. As a result, the vehicle deceleration abruptly fluctuates, which leads to a problem of a sense of incongruity being caused to a passenger in the vehicle.
The invention has been made in view of the problem as mentioned above. It is an object of the invention to prevent vehicle deceleration from abruptly fluctuating during a shift between a cooperative braking mode and a single braking mode regardless of fluctuations in friction characteristics of a frictional member of a frictional braking device. It is a further object of the invention to prevent such vehicle deceleration by correcting one of a frictional braking control amount and a regenerative braking control amount on the basis of a relationship between a reference degree of vehicle deceleration and an actual degree of vehicle deceleration.
Thus, in various exemplary embodiments, the invention provides a vehicular braking control apparatus comprising a mode controller, a calculator, and a corrector. The mode controller controls frictional braking by a frictional braking device and controls regenerative braking by a regenerative braking device and controls shifting between a cooperative braking mode, realized by frictional braking and regenerative braking, and a single braking mode, realized by frictional braking, in case of necessity. The calculator calculates a correction coefficient for one of a frictional braking control amount and a regenerative braking control amount on the basis of an actual degree of vehicle deceleration and a reference degree of vehicle deceleration based on an amount of braking operation performed by a driver during braking in the single braking mode. The corrector corrects the control amount based on the correction coefficient.
According to a further aspect of the invention, a braking control method is provided for a vehicular braking control apparatus that performs frictional braking by a frictional braking device and regenerative braking by a regenerative braking device. Control is performed by shifting between a cooperative braking mode, realized by frictional braking and regenerative braking, and a single braking mode, realized by frictional braking, in case of necessity. The braking control method comprises the steps of calculating a correction coefficient for one of a frictional braking control amount and a regenerative braking control amount on the basis of an actual degree of vehicle deceleration and a reference degree of vehicle deceleration based on an amount of braking operation performed by a driver during braking in the single braking mode, and correcting the control amount based on the correction coefficient.
According to various exemplary embodiments of the vehicular braking control apparatus and the braking control method of the present invention, the correction coefficient for the frictional braking control amount or the regenerative braking control amount is calculated on the basis of the actual degree of vehicle deceleration and the reference degree of vehicle deceleration based on the amount of braking operation performed by the driver during braking in the single braking mode, and the frictional braking control amount or the regenerative braking control amount is corrected based on the correction coefficient. Therefore, even if the friction characteristics of the frictional member deviate from predetermined characteristics, the effect of the deviation in the friction characteristics of the frictional member can be reduced by correcting the frictional braking control amount or the regenerative braking control amount. Hence, the braking force for the entire vehicle can be reliably prevented from abruptly changing during a shift between the cooperative braking mode and the single braking mode. Also, a passenger in the vehicle can be reliably prevented from feeling a shock due to abrupt fluctuations in vehicle deceleration resulting from such abrupt changes in the braking force for the entire vehicle.